This invention pertains to a ceramic matrix composite and a method for preparing the ceramic matrix composite. The ceramic matrix composite comprises a crystalline silicon carbide fiber coated with an interfacial coating in a matrix produced by polymer impregnation. The presence of the crystalline silicon fiber in the matrix allows the matrix to be formed using higher pyrolysis temperatures thereby resulting in a matrix with improved mechanical properties and corrosion resistance.
Ceramic matrix composites and methods for their production are will known in the art. One such method for producing the composites is a polymer infiltration process wherein ceramic fibers, typically coated with an interfacial coating, are impregnated with a ceramic matrix precursor. The impregnated fibers are then formed into the desired shape and cured. The cured shape is then heated to a temperature of about 800xc2x0 C. to 1300xc2x0 C. for a time effective to convert the ceramic matrix precursor into a ceramic. If desired, the composites can be reimpregnated with the ceramic matrix precursor one or more times until the desire porosity of the matrix is achieved.
The temperature at which the molded part is heated to convert the ceramic matrix precursor to the ceramic (xe2x80x9cpyrolysisxe2x80x9d or xe2x80x9cceramificationxe2x80x9d temperature) has in the past been limited to about 1300xc2x0 C. Heating to higher temperatures is desired because crystallization of the matrix phase will occur and thus result in an improved composite. However, heating to a higher pyrolysis temperature causes shrinkage and a loss of mechanical properties due to degradation of the fiber in the matrix.
For example, U.S. Pat. No. 4,460,638 to Haluska discloses a method of making a CMC using a ceramic fiber and a silazane polymer. The polymer is ceramified at a temperature of at least 1000xc2x0 C, preferably at 1200xc2x0 C. U.S. Pat. No. 4,460,639 to Chi et al. discloses a method of making a CMC using a ceramic fiber and organopolysiloxane resins. The polymer is ceramified at a temperature of at least 1 000xc2x0 C., preferably at 1200xc2x0 C. U.S. Pat. No. 4,642,271 to Rice discloses a method of making a CMC using a ceramic fiber coated with BN in a SiC polymer based matrix. The polymer pyrolysis is carried out at a temperature of about 1000xc2x0 C. U.S. Pat. No. 5,067,999 to Streckert et al. discloses a method of making a CMC using a carbon based fiber cloth coated with a boron bonding agent and a polysilane. The polysilane is ceramified at a temperature of 700xc2x0 C. to 1000xc2x0 C. U.S. Pat. No. 5,318,930 to Leung et al. discloses a method of making a CMC using a ceramic fiber and cyclosiloxane monomers containing a vinyl group and/or hydride group. The ceramification is carried out at a temperature of 800xc2x0 C. to 1400xc2x0 C. U.S. Pat. No. 5,725,828 discloses a method for producing a CMC from a modified hydrogen silsesquioxane resin and a coated ceramic fiber. The polymer ceramification is carried out at a temperature of at least 1000xc2x0 C., preferably at 1200xc2x0 C. U.S. Pat. No. 5,707,471 to Petrak et al discloses a method for making a CMC using a coated ceramic fiber and a curable preceramic polymer. The polymer ceramification is carried out at a temperature of at least 1000xc2x0 C., preferably 1200xc2x0 C. None of these references teach pyrolysis of the polymer at a temperature of greater than 1450xc2x0 C.
U.S. Pat. No. 5,145,812 to Arai et al. discloses a method of forming a molded body of silicon-nitride based ceramic by hot pressing at a temperature of 700xc2x0 C. to 2500xc2x0 C. Arai et al. does not teach the use of crystalline silicon carbide fibers or the formation of the composites by polymer impregnation.
It has now been found when a crystalline silicon carbide fiber is incorporated into the matrix composite, that the preceramic polymer matrix may be processed to a ceramic at a higher temperature causing crystallization of the matrix phase. The result is a ceramic matrix composite that has improved mechanical properties and corrosion resistance.
It is therefore an object of this invention to provide a method for producing ceramic matrix composites comprising a crystalline silicon carbide fiber coated with an interfacial coating in a crystal containing ceramic matrix that has improved mechanical properties and corrosion resistance.
This invention pertains to a method for producing ceramic matrix composites comprising impregnating crystalline silicon carbide fibers coated with an interfacial coating with a ceramic matrix precursor comprising a curable silicon-containing preceramic polymer; forming the impregnated fibers into the desired shape; curing the shape; and thereafter heating the cured shape to a temperature of greater than 1450xc2x0 C to 1800xc2x0 C. for a time effective to convert the ceramic matrix precursor into a crystal containing ceramic. A densification step comprised of reimpregnation and ceramification may be optionally carried out one or more times until the desired porosity/density of the ceramic matrix composite is achieved.